Diffractive color separation filter

Abstract

The use of a rhomboid-shaped phase grating to derive, from polychromatic input light, plus-one, minus-one and zero diffraction order output beams each of a different independent color.

What is claimed: 
     
       1. A diffractive color separation filter responsive to illumination with polychromatic input light having a given spectrum of free-space wavelength λ for deriving respective angularly-separated zero diffraction order, plus-one diffraction order, and minus-one diffraction order output light beams, wherein the wavelength spectrum of each of said respective output light beams is significantly different from one another, said filter comprising; a phase grating defined by periodically-spaced rhomboid-shaped diffraction elements having a spacing period d, an optical amplitude a, an aspect ratio b and a slant angle β with respect to the normal to said grating, wherein the respective values of λ, d,a,b, and β are related by the following expressions:   2<d/λ<6   0.5<a/λ<5   0.1<b<0.9   0<β<2λ/d.   
     
     
       2. The filter defined in claim 1, wherein said phase grating is formed by a surface relief pattern in a given material, said surface relief pattern defining said periodically-spaced, rhomboid-shaped diffraction elements. 
     
     
       3. The filter defined in claim 2, wherein said surface relief pattern is formed by a reflective surface of said given material, wherein said surface relief pattern is in contact with a medium having an index-of-refraction n through which said input light and said output light travel, and wherein the physical amplitude a' of said surface relief pattern is substantially equal to a/2n, whereby said phase grating operates as a reflective grating. 
     
     
       4. The filter defined in claim 3, wherein said medium is air having index-of-refraction n substantially equal to one. 
     
     
       5. The filter defined in claim 2, wherein: said given material is transmissive and has an index-of-refraction n 1 , greater than one, through which said input light travels; said surface relief pattern is in contact with a medium having an index-of-refraction n, that is different from n 1 , through which said output light beams travel; and the physical amplitude a' of said surface relief pattern is substantially equal to a/|n 1  -n|, whereby said phase grating operates as a transmissive grating. 
     
     
       6. The filter defined in claim 5, wherein said medium comprises an overcoat material having an index-of-refraction n, greater than one, but different from that of n 1 . 
     
     
       7. The filter defined in claim 5, wherein said medium comprises air having an index-of-refraction n substantially equal to one. 
     
     
       8. The filter defined in claim 7, wherein said given spectrum extends from 400 to 700 nanometers, d is substantially equal to 1.4 micrometers, a' is substantially equal to 2.07 micrometers, b is substantially equal to 0.3 and β is substantially equal to 15°. 
     
     
       9. Optical apparatus responsive to illumination thereof with input polychromatic light having a given spectrum of free-space wavelength λ for deriving three substantially independent color signals representing said given spectrum, said apparatus comprising: a phase grating defined by periodically-spaced diffraction elements having a spacing period d, an optical amplitude a, an aspect ratio b and a slant angle β with respect to the normal to said grating, wherein the respective values of λ,d,a,b, and β are related by the following expressions: 2<d/λ<6   0.5<a/λ<5   0.1<b<0.9   0<β<2λ/d;     said phase grating deriving respective angularly-separated zero diffraction order, plus-one diffraction order, and minus-one diffraction order output light beams in response to the illumination thereof with said input polychromatic light, wherein the wavelength spectrum of each of said respective output light beams is significantly different from one another; and   detection and matrixing means responsive to said respective output light beams for deriving each of said three independent color signals in accordance with said different spectrums of said respective output light beams.   
     
     
       10. The optical apparatus defined in claim 9, wherein: said phase grating comprises a surface relief pattern embossed in a transmissive material having an index-of-refraction of substantially 1.5, which surface relief pattern is in contact with air;   said given spectrum extends from 400 to 700 nanometers, d is substantially 1.4 micrometers, the physical amplitude a' of said rhomboid-shaped diffraction elements is substantially 2.07 micrometers, b is substantially 0.3 and β is substantially 15°; and   said detection and matrixing means includes first detection means for detecting said zero diffraction order output light beam to derive a first detected signal, second detection means for detecting said plus-one first diffraction order output light beam to derive a second detected signal, third detection means for detecting said minus-one first diffraction order output light beam to derive a third detected signal, and subtraction means for subtracting said second detected signal from said third detected signal to derive a difference signal, said first detected signal constituting a color signal representing green, said second detected signal constituting a color signal representing red and said difference signal constituting a color signal representing blue.   
     
     
       11. The apparatus defined in claim 10, wherein said apparatus comprises a color television camera.